The present invention relates in general to the control of solenoid valves and regards in particular a method for determining the instant in which the movable element of a solenoid valve reaches the end-of-travel position following upon energization of the solenoid. In a solenoid valve it is possible to displace the movable part having the function of open/close element between two end-of-travel positions corresponding to the open condition and to the closed condition of the valve, through the application of an appropriate profile of driving current to the solenoid.
The method according to the invention is applicable both to normally open solenoid valves, in which said end-of-travel position corresponds to the closed condition of the valve, and to normally closed solenoid valves, in which said end-of-travel position corresponds to the open condition of the valve.
The measurement of the time that elapses between the instant of energization of the solenoid and reaching of the end-of-travel position by the open/close element (activation time) assumes a fundamental importance for the purposes of a robust control of the valve. Said need is, for example, in particular felt in the case of control systems of internal-combustion engines with electro-hydraulic actuation of the intake and/or exhaust valves of the engine. In said applications, it is extremely important to keep under control the times of actuation (understood as times of opening or closing) of the solenoid valves that regulate passage of the oil in the device for actuation of the engine valves.
A particularly important application of the invention is aimed at the electro-hydraulic system for control of the valves of an internal-combustion engine of the type referred to as UNI-AIR, proposed by the present applicant (see, for example EP 1 653 057 A1), in which corresponding to each engine valve there is a pressurized hydraulic chamber that transmits to the engine valve the movement of a tappet actuated by the camshaft of the engine, and in which said pressurized chamber communicates with an exhaust via a normally open solenoid valve. When the solenoid of the solenoid valve is energized, the aforesaid communication is interrupted, and the aforesaid pressurized chamber hydraulically transmits the movements of the actuation cam to the engine valve. When the solenoid is de-energized, the fluid under pressure is discharged from the aforesaid chamber, so as to cause rapid closing of the engine valve as a result of the respective return spring, thus rendering the engine valve independent of its actuation cam.
There have already been proposed methods for determining the instant in which the movable element of a solenoid valve reaches an end-of-travel position (see, for example, the documents Nos. WO-A-9413991 and EP 1 533 506 A2, the latter being filed in the name of the present applicant), where said instant is identified by analysis of the profile of the current and/or of the voltage for supplying the solenoid.